62 research outputs found
Dynamics of Water Entry
The hydrodynamics associated with water-entry of spheres can be highly
variable with respect to the material and kinematic properties of the sphere.
This series of five fluid dynamics videos illustrates several subtle but
interesting variations. The first series of videos contrasts the nature of
impact between a hydrophilic and hydrophobic sphere, and illustrates how
surface coating can affect whether or not an air cavity is formed. The second
video series illustrates how spin and surface treatments can alter the splash
and cavity formation following water entry. The spinning sphere causes a wedge
of fluid to be drawn into the cavity due to the no-slip condition and follows a
curved trajectory. The non-spinning sphere has two distinct surface treatments
on the left and right hemispheres: the left hemisphere is hydrophobic and the
right hemisphere is hydrophilic . Interestingly, the cavity formation for the
half-and-half sphere has many similarities to that of the spinning sphere
especially when viewed from above. The third video series compares two
millimetric nylon spheres impacting at slightly different impact speeds (Uo =
40 and 45 cm/s); the faster sphere fully penetrates the free surface, forming a
cavity, whereas the slower sphere does not. The fourth series shows the
instability of an elongated water-entry cavity formed by a millimetric steel
sphere with a hydrophobic coating impacting at Uo = 600 cm/s. The elongated
cavity forms multiple pinch-off points along its decent. Finally, a millimetric
steel sphere with a hydrophobic coating breaks the free surface with an impact
speed of Uo = 350 cm/s. The cavity pinches-off below the surface, generating a
Worthington jet that pinches into droplets owing to the Rayleigh-Plateau
instability.Comment: American Physical Society Division of Fluid Dynamics Gallery of Fluid
Motion Video Entry Replaced previous version because abstract had LaTex
markup and was too lon
Quantitative Flow Field Imaging about a Hydrophobic Sphere Impacting on a Free Surface
This fluid dynamics video shows the impact of a hydrophobic sphere impacting
a water surface. The sphere has a mass ratio of m* = 1.15, a wetting angle of
110 degrees, a diameter of 9.5 mm, and impacts the surface with a Froude number
of Fr = 9.2. The first sequence shows an impact of a sphere on the free surface
illustrating the formation of the splash crown and air cavity. The cavity grows
both in the axial and radial direction until it eventually collapses at a point
roughly half of the distance from the free surface to the sphere, which is
known as the pinch-off point. The second set of videos shows a sphere impacting
the free surface under the same conditions using Particle Image Velocimetry
(PIV) to quantify the flow field. A laser sheet illuminates the mid-plane of
the sphere, and the fluid is seeded with particles whose motion is captured by
a high-speed video camera. Velocity fields are then calculated from the images.
The video sequences from left to right depict the radial velocity, the axial
velocity, and the vorticity respectively in the flow field. The color bar on
the far left indicates the magnitude of the velocity and vorticity. All videos
were taken at 2610 fps and the PIV data was processed using a 16 x 16 window
with a 50% overlap.Comment: American Physical Society Division of Fluid Dynamics 2008 Annual
Meeting Replaced previous version because abstract had LaTex markup and was
too long, missing periods on middle initial of first two name
Cavity dynamics of water entry for spheres and ballistic projectiles
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Includes bibliographical references.The free surface impact of solid objects has been investigated for well over a century. This canonical problem has many facets that may be studied: object geometry, surface treatment, and diameter; impact speed and angle; and fluid viscosity and surface tension. The problem is further enriched with the consideration of varying mass ratios and rotational velocities. This thesis uses advanced high-speed imaging and visualization techniques to discover underlying physics and further our understanding of these phenomena through improvements to analytical solutions describing criterion such as cavity formation, depth of deep seal, and trajectory for all impact parameters studied. The topic is extended to the impact of high-speed projectiles or bullets. Through experimentation the trajectory, cavity size, and forces acting on the projectiles are elucidated. Experimentation coupled with improvements to an existing cavitation model lead to an improved bullet design that forms a narrower cavity and achieves higher speeds. Industrial applications include ship slamming, extreme waves and weather on oil platforms, sprayed adhesives, paint aerosols and ink jet printing. In the field of naval hydrodynamics there is particular interest as these problems relate to the study of the water entry of mines and bullets, and the underwater launching of torpedos and missiles. Physical insight can also be applied to sports performance research relating to the water entry of athletes, reducing drag of swimmers near the free surface, decreasing cavity formation for divers, and the entry and exit of oars in rowing.(cont.) This thesis examines the effect of several key parameters on the water entry physics of spheres at relatively low Froude numbers including: hydrophobic vs. hydrophilic surfaces, mass ratio and rotational velocity. Physical models that predict the depth of deep seal and the effect of dynamic and static wetting angle on cavity formation will be discussed. Theories are derived from physical parameters witnessed through high-speed video image sequences using advanced image processing techniques. New phenomena have been witnessed via these techniques including a wedge of fluid that crosses the cavity in the case of transverse rotational velocity. Furthermore, the images reveal the forces acting on the sphere through the entire trajectory, which adds valuable information for future theoretical models. The discussion continues with the water entry of bullets, which produce water vapor cavities large enough to engulf the projectile (i.e. supercavitation). The effects of speed, geometry and angle of attack on the formation of the subsurface cavity are analyzed through an improved physical model and full scale experimentation. The analytical model is then used to improve the design of projectile geometry to allow for more efficient travel inside the cavity and experimentally validated.by Tadd Trevor Truscott.Ph.D
Error sources in three-dimensional microscopic light field particle image velocimetry
Three-dimensional (3D) microscopic velocimetry methods have been increasingly developed in recent years to meet the measurement demands of microfluidic systems. As all 3D microscopic velocimetry techniques involve reconstructing a volume from two-dimensional (2D) sensor(s), sources of uncertainty arise that are unique from 2D velocimetry methods. This study discusses the error sources associated with a recently developed microscopic light field particle image velocimetry (LFPIV) method. The LFPIV technique combines altered optical hardware with postcapture computation to reconstruct 3D volumes. A microlens array placed at the intermediate image plane of an infinity corrected objective captures the directionality of light rays, which may then be reparameterized to form a 3D focal stack. The error sources of LFPIV are typical of image-based 3D reconstruction. We group these errors into four categories: experimental setup, calibration, 3D reconstruction, and velocimetry. All 3D microscopic particle image velocimetry methods introduce additional complexity into the experimental setup and the particular challenges of LFPIV will be highlighted. Calibration errors arise from imperfect mapping between the 3D world and LFPIV instrument (optics and computation inclusive). The most unique error source in 3D velocimetry methods stems from 3D reconstruction. Objects are typically estimated with large error on the depth dimension. We discuss the magnitude of this error for LFPIV and its dependency on instrument design. Methods for improving reconstruction quality, such as 3D deconvolution and focus-based thresholding, are assessed. Most importantly, the impact of these error sources on uncertainty, accuracy, and resolution of velocity measurements is quantified using data from a microchannel flow field, a numerical model and simulated data. Comparisons to existing techniques are made whenever possible
Drop on a Bent Fibre
Inspired by the huge droplets attached on cypress tree leaf tips after rain,
we find that a bent fibre can hold significantly more water in the corner than
a horizontally placed fibre (typically up to three times or more). The maximum
volume of the liquid that can be trapped is remarkably affected by the bending
angle of the fibre and surface tension of the liquid. We experimentally find
the optimal included angle () that holds the most water.
Analytical and semi-empirical models are developed to explain these
counter-intuitive experimental observations and predict the optimal angle. The
data and models could be useful for designing microfluidic and fog harvesting
devices
On the Threshold of Drop Fragmentation under Impulsive Acceleration
We examine the complete landscape of parameters which affect secondary
breakup of a Newtonian droplet under impulsive acceleration. A Buckingham-Pi
analysis reveals that the critical Weber number for a
non-vibrational breakup depends on the density ratio , the drop
and the ambient Ohnesorge numbers. Volume
of fluid (VOF) multiphase flow simulations are performed using Basilisk to
conduct a reasonably complete parametric sweep of the non-dimensional
parameters involved. It is found that, contrary to current consensus, even for
, a decrease in has a substantial
impact on the breakup morphology, motivating plume formation. In addition to
, also affects the balance between pressure differences
between a droplet's pole and its periphery, and the shear stresses on its
upstream surface, which ultimately dictates the flow inside the droplet. This
behavior manifests in simulations through the observed pancake shapes and
ultimately the breakup morphology (forward or backward bag). All these factors
affecting droplet deformation process are specified and theories explaining the
observed results are provided. A plot
is provided to summarize all variations in observed
due to changes in the involved non-dimensional parameters. All observed
critical pancake and breakup morphologies are summarized using a phase diagram
illustrating all deformation paths a droplet might take under impulsive
acceleration. Finally, based on the understanding of process of bag breakup
gained from this work, a non-dimensional parameter to predict droplet breakup
threshold is derived and tested on all simulation data obtained from this work
and all experimental data gathered from existing literature
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